1. Field of the Invention
This invention relates to analog circuit tests, and particularly to circuits within a tested circuit for testing the tested analog circuit using a digital tester.
2. Description of Background
Before our invention external analog testers were used by IBM to test existing PLL designs which are sensitive to the characteristics of the loop filters. Most are sensitive to the leakage in the capacitors of that filter.
Prior solutions require using a dedicated analog test I/O, perhaps with an analog multiplexer to select which analog function is to be tested through a shared pin. As technology has progressed, it has become increasingly difficult to design such a solution. The additional leakage introduced by an analog multiplexer can disrupt the circuit activities. Adding a dedicated analog test I/O can introduce noise into the circuit. In addition, most manufacturing test instrumentation is not equipped to measure small analog values.